There are many audio products on the market today that are voltage supply limited, i.e., operate with a fixed voltage supply source. For example, both far and near can there be found battery powered audio products such as portable radios, tape and CD-players, "Boom Boxes", SONY Walkman.RTM., etc. It is desirable for all such audio products, whether portable or not, to produce a low distortion acoustic output that is sufficient to be heard by the listener regardless of the volume and power levels. However, inherent technical problems exist in achieving low distortion output power of audio equipment because of the following reasons: 1) electrical power into a speaker is determined by the relation V.sub.rms.sup.2 /R.sub.speaker, i.e., the root mean square voltage supplied to the speaker divided by the speaker impedance. As V.sub.rms is limited, then the acoustic output power is limited. The maximum low distortion sin wave that can be applied to a speaker from an electrical source having voltage supply V+ and V- is a peak-to-peak value equal to the voltage supply. Although the maximum rms output value can be higher, it will have a significantly higher value of total harmonic distortion (THD); and, 2) most audio signals have a wide dynamic range and in the case of music, the ratio of maximum audio signal amplitude to average audio signal amplitude may be 2:1 to 10:1 or greater. In order to avoid distortion, the gain of the system must be maintained at a level where the maximum signal will not distort when it runs out of voltage. A consequence of reducing the gain is that the average audio signal amplitude is reduced by a significant amount.
Currently, Automatic Gain Control (AGC) circuitry has been implemented in audio systems to reduce the effect of distortion by monitoring only the output audio signal amplitude and dynamically reducing the signal amplitude if the output audio signal amplitude exceeds a predetermined AGC (voltage or current) trigger level. A drawback of the AGC implementation is that it prevents the audio signal of interest from exceeding a fixed amplitude level and additionally, reduces average audio signal amplitude.
It would thus be highly desirable to provide an audio system having the capability of producing a maximum low distortion acoustic power for audio signals without reducing average audio signal amplitude.
Moreover, it would thus be highly desirable to provide the capability of producing a maximum low distortion acoustic power for audio signals in an audio system having limited voltage supply.